In machine tools and other automated machinery, the work operation involves a force being applied to an object that is to be worked, or a force being received from an external source. In such cases, automated machines require that the moment and external force applied thereto be detected, and a control corresponding to the force and moment be carried out. The external force and moment must be accurately detected in order to carry out the control corresponding to the force and moment with high accuracy.
In view of this, a variety of force sensors have been proposed in the past. Using the detection method as a basis, the force sensors are normally classified into elastic force sensors and balanced force sensors. The elastic force sensor measures force on the basis of the amount of deformation in proportion to the external force. The balanced force sensor measures force according to a balance with a known force.
The theoretical structure of another known force sensor involves a plurality of strain resistance elements being provided to a portion of a load cell, which elastically deforms in response to an external force. When an external force is applied to the load cell of the force sensor, electric signals according to the degree of deformation (stress) of the load cell are outputted from the plurality of stress resistance elements. The force of two or more components or the like applied to the load cell can be detected on the basis of these electric signals. The measurement of stress generated in the force sensor is calculated on the basis of the electric signals described above.
A six-axis force sensor is one known type of force sensor. The six-axis force sensor, which is a type of elastic force sensor, has a plurality of strain resistance elements provided to a load cell portion. The six-axis force sensor detects an external force as six axis components, which are divided into stress components (force: Fx, Fy, Fz) in the direction of the three axes (X axis, Y axis, and Z axis) of a Cartesian coordinate system; and torque components (moment: Mx, My, Mz) in the direction of the axes.
Japanese Patent Application Laid-Open Publication No. 2006-125873 (JP 2006-125873 A) discloses a multi-axis force sensor chip and a multi-axis force sensor assembled using this chip.
In the multi-axis sensor chip, a semiconductor manufacturing technique is used to form a plurality of strain resistance elements on a connecting part of a semiconductor substrate (base member) having a prescribed shape and structure. The strain resistance elements is affected by the stress generated in response to the force or moment applied to an operating part of the semiconductor substrate, and the resistance values change. The force or moment applied to the operating part of the semiconductor substrate is calculated by appropriately combining the resistance values of the plurality of strain resistance elements.
In the multi-axis sensor disclosed in JP 2006-125873 A, temperature-compensating resistance elements are provided correspondingly with respect to the plurality of strain resistance elements. The variation characteristics of the resistance values of the strain resistance elements are inherently temperature-dependent, and the corresponding temperature-compensating resistance elements and the stress resistance elements are therefore equally affected by temperature.
A thick film circuit board for a pointing stick is disclosed in Japanese Patent Application Laid-Open Publication No. 2000-267802 (JP 2000-267802 A). A pointing stick is a component used as a signal input device in a notebook computer or the like. A signal input operation is carried out by manipulating a pointing stick in the x, y, and z directions using finger pressure. When a finger manipulates the pillar-shaped body that constitutes the stick, a lower end part of the pillar-shaped body applies a force or moment to a center part (force action point) of the thick film circuit board. When the thick film circuit board deforms, the resistance values of the plurality of resistance bodies on the thick film circuit board are varied, and an input signal is generated. According to the thick film circuit board, four resistance elements are disposed in a point-symmetrical positional relationship around the force action point in order to provide a satisfactory output balance among the axes.
According to the multi-axis force sensor chip disclosed in JP 2006-125873 A, the temperature-compensating resistance elements are provided to locations such that the distance to the locations where the strain resistance elements are disposed (which are regions where the semiconductor substrate does not deform) can be reduced to the greatest extent possible; and such that the temperature conditions can be made equal. This is performed in order to remove the effect of the temperature on the temperature-dependent strain resistance elements. It is thereby possible to compensate for the temperature with high accuracy.
However, even when the temperature-compensating resistance elements are provided to locations where the temperature conditions are the same as those where the strain resistance elements are disposed (non-deforming regions), the initial resistance values (resistance values when no load is present) of the strain resistance elements will actually be irregular. As a result, a problem occurs in that optimal values will not necessarily be obtained even when the temperature-compensating resistance elements are used to perform temperature compensation on the output values of the strain resistance elements.
In the multi-axis force sensor chip, it is desirable for the outputs across the axes to be properly balanced, and a proposal for achieving such a result is disclosed in, e.g., JP 2000-267802 A. However, the structure of the thick film circuit board disclosed in JP 2000-267802 A corresponds to a ceramic substrate. In the case of a semiconductor substrate, the crystal orientation in the semiconductor substrate must be taken into consideration in order to equalize the output resistance values across the axes.
Furthermore, as mentioned in JP 2006-125873 A as well, the temperature-compensating resistance elements are disposed with consideration given to crystal orientation so that the strain sensitivity of the strain resistance elements is considerably higher than the strain sensitivity of the temperature-compensating resistance elements. However, this document does not disclose the concept of determining the layout of the resistance elements in consideration of the crystal orientation from the standpoint of properly balancing the outputs across the axes.